1. Field of the Invention
The invention relates to energy storage devices, such as flywheels, and particularly to high-speed rotary structures having high volumetric and cost efficiency.
2. Description of the Prior Art
The flywheel has been used for centuries as an energy storage device. Since the flywheel is an inertial device governed by the laws of kinetic energy, maximum performance is attained at maximum speed, the performance being generally quadrupled with a two-fold increase in speed. The speed of the rotating body, however, cannot be increased beyond its bursting limit. In the prior art, three general flywheel configurations are predominant, namely, the flat disc type characterized by smooth parallel surfaces between the hub and the periphery; the rim type having a massive peripheral portion secured to the hub by spokes or by a solid wheel portion; and the more recently developed optimized disc.
Materials used to fabricate high-energy flywheels must have large specific strengths (strength/density) to enable the structure to be rotated at a high velocity. High strength steel has ordinarily been chosen as flywheel material. However, the strength/density ratio of an isotropic steel structure is substantially less than that obtainable with modern anisotropic filamentary materials. High strength filaments typically exhibit substantially greater strength-to-density characteristics over the best isotropic materials, such as steel or titanium. Rotary energy storage devices fabricated from these high strength filaments have greatly increased energy densities when compared to isotropic structures. However, the volumetric efficiencies, i.e., watt hours per unit volume of rotor, of most filamentary rotors is low. Further, the cost of these high strength filamentary rotors is often prohibitive for a number of applications.
The present inertial energy storage device offers substantial improvement in usable energy density due not only to the advantageous utilization of the high uniaxial strength of filamentary materials, but also to the increased packaging densities provided by the several embodiments of the invention, thereby improving volumetric efficiency. Further, costs of the several embodiments of the invention are significantly lower than that of pior art filamentary rotors.
The significance of the present energy storage device is best understood by its application to the urban vehicle. Although flywheels have been used in short-range vehicles, such as in the Swiss Oerlikon bus and in the British Gyreacta transmission, those devices produced only about three watt-hours per pound. Thus, energy density of the devices was even lower than that of available lead-acid batteries at the same discharge rate. However, certain characteristics of flywheels caused their use in preference to storage batteries, despite the problems then encountered in the use of flywheel structures. Firstly, the flywheel can be charged and discharged virtually an infinite number of times without degrading performance. Secondly, it can be charged at any reasonable rate. Thirdly, it can be discharged at any rate within the design limitations of ancillary equipment without degrading performance. These capabilities are largely responsible for the proposed use of flywheels in pollution-free urban vehicles. In most previous proposals, the rapid discharge capability of the flywheel has been primarily used to lend increased acceleration power to the vehicle in order to minimize the overall size of the main propulsion power plant. The present energy storage device provides a power plant of sufficient energy density to also enable its economic and practical use as the primary energy source in an urban vehicle.